The present invention relates, in general, to semiconductor circuits and, more particularly, to semiconductor downconverters.
Downconverters are used in radio applications to convert a signal of one frequency to a signal of another frequency. A downconverter typically includes a low noise amplifier (LNA), an image filter, and a mixer circuit. The LNA amplifies a radio frequency (RF) signal which is filtered through the image filter and mixed with a local oscillator (LO) signal in the mixer circuit. The mixing process provides a mixed signal, which has a spectrum comprised of the sum and difference of the frequencies of the RF and LO signals. The higher frequency components in the mixed signal are filtered out leaving an intermediate frequency (IF) component which serves as the output signal of the downconverter. To prevent the RF circuitry and the LO circuitry from interacting with each other, the mixer circuit may use a dual gate field effect transistor or two field effect transistors in a cascode configuration to mix the RF and LO signals.
A common problem with a downconverter is the need to individually supply the low noise amplifier and the mixing transistor with sufficient current for operation. For example, the low noise amplifier and mixing transistor may each require two to three milli-amps of operating current. Circuitry that consumes current at this level may not be suitable for portable applications such as cellular telephones, digital pagers, etc.
Another problem with a downconverter occurs when the mixer circuit is switched off. While the mixer circuit is off, the LO signal may have sufficient power to turn on the mixing transistor in the mixer circuit. A switching circuit in the LO signal path is typically used to block the LO signal from the gate of the mixing transistor during the off period to ensure that the mixing transistor is switched off. However, the switching circuit increases the complexity of the mixer circuit and the size of the integrated circuit chip.
Accordingly, it would be advantageous to have a downconverter and a method for generating an IF signal. It is desirable for the downconverter to have a low power consumption. It would be of further advantage for the downconverter to be fabricated using a minimum chip area.